California's law took effect in 2009. It was passed in response to undercover video aired by the Humane Society. The clip showed workers in a Los Angeles-area slaughterhouse dragging and forcing downed livestock into pens.

The state law sought to regulate how slaughterhouses treated these "non-ambulatory" animals. But the Federal Meat Inspection Act (FMIA) already provided regulations. After the California law went into effect, the National Meat Association brought a lawsuit seeking to overturn the new statute.

The Supreme Court voted unanimously against the California law, in favor of the meat association.

The court found that the FMIA and the California slaughterhouse law were essentially creating additional and different requirements. This is expressly prohibited under the FMIA.

The FMIA regulates downed livestock by classifying them. Animals with disease or conditions are to be slaughtered apart from the food and considered "condemned." Non-ambulatory animals that aren't "condemned" are classified as "suspect" and also slaughtered separately. They are inspected after the slaughter where an inspector will determine if any parts are consumable.

The California statute specifically prohibits a slaughterhouse from purchasing, selling, or receiving downed livestock. Slaughterhouses are also prohibited from selling any parts of downed livestock for human consumption. They are also required to immediately and humanely euthanize animals that are non-ambulatory.

Animal rights groups including the Humane Society disagree with the court's ruling on California's slaughterhouse law. They believe that the federal government still needs to do more to further the humane treatment of downed livestock, according to CNN.